Former Italian prime minister Silvio Berlusconi is being investigated for witness tampering in a trial in July. Photograph: Luigi Mistrulli/Sipa/Rex

Prosecutors in Milan are investigating Silvio Berlusconi for alleged witness corruption following two high-profile trials relating to parties held at his luxurious villa.

Though widely expected, the new investigation will nonetheless come as a blow to the former Italian prime minister as he seeks to shore up his political influence in the wake of his conviction for tax fraud and expulsion from parliament last year.

A total of 45 people, including Berlusconi, his two chief defence lawyers and several young women, are reported to be under investigation in the new inquiry, which was announced by chief prosecutor Edmondo Bruti Liberati.

Prosecutors will be investigating allegations that witnesses who testified on behalf of the 77-year-old in the so-called "Ruby" trial – in which he was accused of paying for sex with an underage sex worker and then abusing his office to cover it up – and a related trial were corrupted.

Lawyers for Berlusconi have previously denied the allegations, and on Thursday the leader of the centre-right Forza Italia party issued a statement reiterating his belief that he is being hounded by a leftwing judiciary bent on his destruction.

"I am here, and am staying here, strongly conscious of the responsibility that comes from the trust and the votes of the people," he said in a statement. "I am staying on the field, more convinced than ever that I must fight on to the end to see the triumph of what I deeply believe in."

He added: "In Italy today we can safely say: 'The law is equally unfair for all citizens.'"

The concerns of the judges in that trial were then echoed in the written reasoning issued by their colleagues in November in a related trial.

In the second trial, the judges said there was evidence to suggest that Berlusconi had paid "money and other benefits" to young women in return for them giving misleading evidence. The payment of €2,500 a month to some of the witnesses "is not an anomaly but an illegal offence", they claimed, describing it as "tampering with evidence".

Berlusconi has claimed the payments are simply an act of benevolence on his part.

In November, his lawyers, Niccolo Ghedini and Piero Longo, who are reportedly among those now being investigated, said the accusations were "totally disconnected from reality and from the facts".

During the Ruby trial, Berlusconi denied having paid Moroccan dancer Karima el-Mahroug, then 17, for sex, insisting the two had never had intimate relations – an assertion supported by her.

The four-times Italian PM said that the soirees he held at his Arcore villa near Milan were simply "elegant dinners", though other witnesses painted a different picture.